Btrfs: reset to appropriate block rsv after orphan operations

While truncating free space cache, we forget to change trans->block_rsv
back to the original one, but leave it with the orphan_block_rsv, and
then with option inode_cache enable, it leads to countless warnings of
btrfs_alloc_free_block and btrfs_orphan_commit_root:

WARNING: at fs/btrfs/extent-tree.c:5711 btrfs_alloc_free_block+0x180/0x350 [btrfs]()
...
WARNING: at fs/btrfs/inode.c:2193 btrfs_orphan_commit_root+0xb0/0xc0 [btrfs]()
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/free-space-cache.c

@@ -190,9 +190,11 @@ int btrfs_truncate_free_space_cache(struct btrfs_root *root,
 				    struct btrfs_path *path,
 				    struct inode *inode)
 {
+	struct btrfs_block_rsv *rsv;
 	loff_t oldsize;
 	int ret = 0;
 
+	rsv = trans->block_rsv;
 	trans->block_rsv = root->orphan_block_rsv;
 	ret = btrfs_block_rsv_check(trans, root,
 				    root->orphan_block_rsv,
@@ -210,6 +212,8 @@ int btrfs_truncate_free_space_cache(struct btrfs_root *root,
 	 */
 	ret = btrfs_truncate_inode_items(trans, root, inode,
 					 0, BTRFS_EXTENT_DATA_KEY);
+
+	trans->block_rsv = rsv;
 	if (ret) {
 		WARN_ON(1);
 		return ret;